1. Field of Invention
The present invention relates to a soft-start device, and more particularly to a soft-start device that has a smooth switching process without any surge, and is capable of prolonging a starting time.
2. Related Art
Soft-start devices may be applied in multiple circuits. Circuits in which an input power must rise slowly to avoid damage to the circuits due to an excessive current surge or an excessive voltage at an initial power on stage, may all use soft-start devices.
Taking a switching regulator as an example, the switching regulator has good voltage conversion efficiency and is therefore usually used for converting large voltage differences and large load current. However, at an initial power on stage, the switching regulator easily results in an excessive current surge or an excessive voltage, thereby possibly damaging the circuit. Therefore, a soft-start device needs to be used at the power on stage to make the power supply voltage rise slowly.
FIG. 1A is a schematic view of a soft-start device in the prior art. In the design of the prior art, the soft-start device includes a current source (I) A10, a capacitor (C) A20, a switch A30, a reference voltage (Vref) A40, and a fixed voltage source (Vbg) A50.
At the beginning, the soft-start device performs an open-loop operation, that is, the switch A30 is in an open state. The capacitor A20 is charged by the current source A10. In the charging process, the capacitor A20 slowly increases the voltage to generate a ramp voltage. Therefore, the reference voltage A40 is increased slowly with the ramp voltage at the same time, so as to achieve the soft-start.
When the ramp voltage approaches a voltage value of the fixed voltage source A50 (here the voltage value of the fixed voltage source A50 may be a bandgap voltage), the soft-start mechanism must end in order to restore the whole system to normal operation. The conventional practice is closing the switch A30 to switch back to a closed loop, such that the reference voltage A40 outputs the voltage value of the fixed voltage source A50, that is, it outputs the bandgap voltage.
However, the switching time point of the switch A30 is hard to control, which easily causes discontinuous surges such that the system is unstable, and may also cause faults in operation. FIG. 1B is a schematic view (I) of a switching waveform of the soft-start device in the prior art. As FIG. 1B demonstrates, if the switch A30 is switched too late the reference voltage (Vref) exceeds the bandgap voltage (Vbg), so as to form an upward surge before returning to the bandgap voltage value (Vbg). Conversely, FIG. 1C is a schematic view (II) of a switching waveform of the soft-start device in the prior art. As FIG. 1C demonstrates, if the switch A30 is switched too early the reference voltage (Vref) does not reach the bandgap voltage (Vbg), so that a gap is generated between the reference voltage A40 and the bandgap voltage.
On the other hand, the soft-start device mainly functions to raise the power supply voltage slowly, so as to avoid damage to the circuit due to the excessive current surge at the initial stage of the power on. The slower the power supply voltage rises, the smaller the voltage rising in a unit time is, and thus the circuit is less likely to be damaged. Therefore, the soft-start device is generally required to have a long soft-start time, such that the power supply voltage may rise gradually. A time required for the soft-start is expressed in the following equation:
      T    s    =            C      I        ×                  V        bg            .      
As the above equation describes, in order to make the soft-start time (Ts) long, the current source (I) must be small and the capacitor (C) must be large. In the conventional practice, a large capacitor is connected externally using an additional pin, so as to achieve increased capacitance. However, in such a practice not only the additional pin and large capacitor are added (resulting in increased cost), but also the externally connected capacitor is difficult to integrate into the IC.
Therefore, the challenge of how to solve the relevant problems of switch switching and soft-start time of the soft-start device in the prior art is an issue pressing for a solution.